This invention is related to electrically held power relays in which a closed relay is to be provided with a reduced power dissipation.
Electrically held relays are operable to close electromagnetic contacts of a controlled circuit by application of a closing current of a given magnitude to the relay coil. Once the relay contacts are closed, the current through the coil required to hold the contacts closed is typically only about one-fourth of that required to achieve closing. Thus, if the same current is supplied from the power supply circuit to the coil, there will be power dissipation in the coil after closing that is not associated with any benefit. Particularly in applications where circuitry is arranged in a compact configuration and yet must be of high reliability, such as in aerospace applications, such power dissipation is desirably to be avoided. Power dissipation can be particularly serious at or above normal voltages as the power is the product of the input voltage and the relay coil current.
Attention has been previously given to relay coils and regulating circuitry for reducing the power dissipation.
FIG. 1 shows a representative example of known prior art in which a relay coil 10 is provided with an economizer circuit 12 for reducing operating current to hold in the relay contacts 14 after closing. The arrangement generally comprises a series circuit comprising the main relay coil 10, a so-called "tail switch" 16 and a voltage regulator. Electronic switch 18 controls the application of current to the circuit branch containing coil 10. The tail switch 16 is simply a pair of contacts electromagnetically actuated off the mechanism operated by the main coil 10 so that late in the travel of that mechanism, such as a plunger-type solenoid, and after sufficient closing current has been supplied to the main coil, the previously closed tail switch is opened. Then resistor R, connected in parallel with the tail switch 16 is in circuit with the main coil and reduces the current through the coil 10. Alternatively, or additionally, there may be a holding coil L connected across the tail switch 16 also serving to reduce steady-state power dissipation.
It is desired to eliminate the tail switch 16 as it is an additional mechanical element with some inherent reliability drawbacks or requirements for careful adjustment during production and assembly.
In accordance with the present invention a circuit is provided in which the inductance of the power relay coil is utilized as the choke of a switching current regulator. Means are provided in the regulating circuitry for sensing the current in the relay coil and comparing it with a reference so that, upon achievement of the required closing current level, supply current is no longer supplied to the relay coil but its required holding current is maintained by the inductance of the coil itself and upon decay of the relay current down to near the holding current minimum level, the supply current is switched back into the relay coil for a brief period.
The arrangement preferably includes transistor or solid-state switches and a voltage comparator for maintaining high reliability and also includes an additional transistor means for speeding up the reduction of coil current upon removal of the DC supply to result in fast opening of the relay.
Arrangements in accordance with this invention can reduce steady-state power dissipation over prior art such as that of FIG. 1 by an order of magnitude or more as well as provide greater reliability by elimination of the mechanical tail switch.